1. Field of the Invention
The present invention relates to a method and an apparatus for fire extinguishing which are particularly suitable for extinguishing burning liquids.
2. Prior Art
To extinguish a fire, the temperature has to be reduced, the free access to the oxygen of the air has to be prevented, or the burning material has to be removed. Each of the aforementioned factors is sufficient for extinguishing the fire but, depending on the circumstances, one usually tries to use the two first-mentioned in combination.
The most common and most universal extinguishing means is water, the ability of which to extinguish is due to its cooling effect. It is however usually not suitable for extinguishing burning liquids. Nowadays various foams are used for fire extinguishing, for the generation of which water is also needed. The ability of the foams to extinguish fire is due to their cooling effect and partly to their smothering effect, i.e. they prevent the free access of the oxygen of the air, which is necessary for the burning. The foams are quite suitable for extinguishing many burning liquids. The ability of powders to extinguish fires is due to their prevention of the free access of the air to the object of the fire. Various oxygen-free gases are also used for fire extinguishing. Their ability to extinguish fire is due to the fact that they, being heavier than the air, displace the air from the object of the fire or mix with the air, whereby the proportion of the oxygen becomes so small that burning is impossible.
The use of the abovementioned fire extinguishing means is restricted or hampered by the following circumstances.
Burning liquids and eletrical fires cannot be extinguished by water with the exception of a few cases. Unskillfully used or because of difficult conditions, the damages caused by the water can be worse than those caused by the fire. Common water damages are interruption of production for long periods and damage to the buildings. Lack of water as a rule means that a small fire turns into a big fire, because the firebrigade can only carry small quantities of water. The oil harbours are usually located in such a manner that water for extinguishing is not obtainable from the sea even if they are near it. The most common reason for that are shallow shores or that there is not enough space for sufficient fire-fighting equipment at the source of the water supply. Usually the size of the water pipes and the available amount of water are not large enough to supply water sufficiently for extinguishing burning oil cisterns and entrenchments and for cooling adjacent cisterns.
If there is not water, no foam can be produced. The heavy protein base foams need plenty of water because the portion of the foaming liquid in the extinguisher-foam is only 8%. If protein base foams come into contact with air in their storage containers, the decay process starts immediately and after a year they are usually spoiled. The half-life of the foams is about 5-10 min., whereas 30 min. is required. The light synthetic foams cause intensive corrosion. On the other hand, they move easily by air currents because of their lightness. The foams cannot, whether heavy or light, prevent burning liquid from moving or flowing, because they do not absorb the burning liquid but aim at forming a foam layer on them in order to prevent air from mixing with the vapours from the burning liquid. The heavy foams can by means of a fire-engine be thrown only about 25 m, which means that its operating distance is rather short. Light foams can, owing to their lightness, only be transferred about 12 m at the most by means available today.
Powders are usually used only indoors and they are generally used as extinguishing means in preliminary fire extinguishers. A fire foam truck can actually be used only once and has to be refilled when empty. A refill takes at present more than 30 min. and during this time the fire has already increased so much that it is impossible to extinguish it with powder. Powders do not have any cooling effect.
Gases can successfully be used indoors only or in the preliminary stages of small fires. They have no cooling effect. Most gases which are used for extinguishing purposes are poisonous or become poisonous when they decompose.